Compounds which, owing to their typical terbium emission with a maximum wavelength of about 541-543 nm, emit in a narrow band are preferably used as green components in fluorescent lamps, in particular compact fluorescent lamps and triphosphor tube fluorescent lamps. These include the luminescent substances cerium magnesium aluminate, activated by Tb (CAT) according to U.S. Pat. No. 4,837,481; lanthanum phosphate, activated by Ce, Tb (LAP) according to GB 2 124 243 and U.S. Pat. No. 4,891,550; lanthanum phosphate silicate, activated by Ce, Tb (LAPS) according to U.S. Pat. No. 4,629,582; and Y.sub.2 SiO.sub.5, activated by Ce, Tb according to EP 0 037 688 as the most important members. All these luminescent substances are distinguished by high thermal stability and luminous efficiency. The high production costs caused by the required preparation temperatures of 1300.degree. C. to 1600.degree. C. are disadvantageous.
A further luminescent substance which has its emission maximum at 542 nm is gadolinium magnesium pentaborate, activated by Ce, Tb, which is described in EP 0 023 068. Characteristic of the luminescent substance CBT is the relatively low production temperatures of only slightly above 1000.degree. C. Owing to its high stability and its good emission properties, it is equivalent in use to the luminescent substances CAT, LAP, LAPS and Y.sub.2 SiO.sub.5, activated by Ce, Tb. But the luminescent substance CBT owns the disadvantage, like most of the borate phosphors with their relative coarse-grained structure, irregular graining and the difficult way of production.
Another green-emitting luminescent substance consisting of (La,Ce,Tb) PO.sub.4 became known by U.S. Pat. No. 4,764,301. After the substitution of one part of the phophoric acid for a boric acid group the temperature-versus-luminescence characteristics of this substance could be improved.
Futher luminescent substances became known by EP 0 057 026, based on rare earth metal metaborates of the formula (Y,La).sub.1-x-y-z Ce.sub.x Gd.sub.y Tb.sub.z (Mg,Zn).sub.1-p Mn.sub.p B.sub.5 O.sub.10, as well in U.S. Pat. No. 4,319,161 of the formula (Y,La).sub.1-x-y-z Ce.sub.x Gd.sub.y Tb.sub.z (Mg,Zn).sub.1-p Mn.sub.p B.sub.5 O.sub.10, in U.S. Pat. No. 4,602,188 of formula Ln(Mg,Zn,Cd)B.sub.5 O.sub.10, activated by Ce, Mn, in which Ln may be the elements Y, La, and/or Gd and in U.S. Pat. No. 5,043,308 of the formula LnAl.sub.1.67-0.67X (B.sub.4 O.sub.10)O.sub.X, in which Ln is an element of the group IIIB. The common disadvantage of these luminescent substances is also the difficult processing way.
It is the object of the present invention to develop a luminescent substance which has further improved emission and processing properties and can be used both in straight fluorescent lamps and in compact energy-saving lamps. The present invention relates to variously activated luminescent metaborate substances having luminescence and particles properties improved in comparison with conventional luminescent substances and outstanding stability under the influence of short-wave UV radiation so that they are preferably suitable for use in luminescent screens, in particular in low-pressure mercury vapor discharge lamps of all types and designs. The luminescent substances according to the present invention are based on special rare earth metal metaborates with the monocline pentaborate structure and of the formula EQU (Y,La).sub.1-x-y-z Ce.sub.x Gd.sub.y Tb.sub.z (Mg,Zn,Cd).sub.1-p Mn.sub.p B.sub.5-q-s (Al,Ga).sub.q (X).sub.s O.sub.10,
in which X is Si, Ge, P, Zr, V, Nb, Ta, W or two or more than two of the stated elements and p, q, s, x, y and z are alternatively .ltoreq.1. It is noted that in the above formula (Y,La) includes Y and/or La; (Mg,Zn,Cd) includes Mg and/or Zn and/or Cd; and (Al,Ga) includes Al and/or Ga.
The luminescent substances may be used individually or as a mixture of a plurality thereof for the luminescent layer of screens or in low-pressure mercury discharge lamps.